When operation of a brushless direct current motor is controlled, multiple Hall elements for detecting a rotation position of a rotor are arranged on a stator side such that the Hall elements are arranged by each electric angle of 60 degrees or 120 degrees. Based on detected position signals from the Hall elements, an energizing timing to the stator coil is determined. However, it is required to include a power source wiring for energizing the Hall element and a wiring for outputting a detection signal. Accordingly, a structure of the motor becomes complex.
To simplify the structure, a motor driving device having no sensor is proposed in, for example, JP-A-S62-123979. The device detects positioning information of a rotor based on an induced voltage, which is generated in the stator coil when the rotor is rotated, without using a sensor such as a Hall element.
FIG. 5 shows a motor driving device 1 for driving a fan motor of a radiator mounted in a vehicle according to a related art. The device 1 is energized from a battery 2 of the vehicle so that a driving power is supplied from the battery 2. A brushless DC motor 3 is energized through an inverter 4. The inverter 4 includes, for example, six power MOSFETs 5a-5f, which are coupled with one another with a three-phase bridge connection. Each phase output terminal in the inverter 4 is connected to a corresponding stator coil 6U, 6V, 6W of the motor 3.
The inverter 4 is controlled with a control portion 7, which includes a micro-computer or a logic circuit. The control portion 7 inputs a driving signal to a gate of each MOSFET 5a-5f through a gate driver 8. A position of a rotating rotor of the motor 3 is detected by a position detection circuit 9. A position detection signal is inputted to the control portion 7. The position detection circuit 9 includes three low pass filters 10U, 10V, 1W, three buffer amplifiers 11U, 11V, 11W and three comparators 12U, 12V, 12W. Each low pass filters 10U, 10V, 10W is mainly composed of a capacitor C and a resistor R. Each comparator 12U, 12V, 12W compares an output signal of the buffer amplifier 11U, 11V, 11W with a virtual neutral point potential, i.e., VNP potential. The input terminal of each low pass filter 10U, 10V, 10W is connected to a common connection point between two resistors R1U, R2U, R1V, R2V, R1W, R2W. The two resistors R1U, R2U, R1V, R2V, R1W, R2W divide an output voltage of each phase output terminal of the inverter 4.
FIGS. 6A to 6I show a voltage waveform in each portion when the motor 3 is energized through the inverter 4. Specifically, FIG. 6A shows a voltage waveform of a U phase in the induced voltage, FIG. 6B shows a voltage waveform of a V phase in the induced voltage, and FIG. 6C shows a voltage waveform of a W phase in the induced voltage. FIG. 6D shows a voltage waveform of a U phase in a signal after passing through the filter 10U, 10V, 10W, FIG. 6E shows a voltage waveform of a V phase in the signal after passing through the filter 10U, 10V, 10W, and FIG. 6F shows a voltage waveform of a W phase in the signal after passing through the filter 10U, 10V, 10W. FIG. 6G shows a voltage waveform of a EU component in the comparator output signal, FIG. 6H shows a voltage waveform of a EV component in the comparator output signal, and FIG. 6I shows a voltage waveform of a EW component in the comparator output signal. Here, the comparator output signal corresponds to the position detection signal. When the motor 3 starts to activate, the control portion 7 provides a predetermined pattern for energizing. After the motor 3 rotates, the induced voltage generated in each stator coil 6U, 6V, 6W appears as a terminal voltage of the coil 6U, 6V, 6W. Since a switching waveform caused by a PWM control method is overlapped with the terminal voltage of the coil 6U, 6V, 6W, the switching waveform is removed by the low pass filter 10U, 10V, 10W. Thus, an induced voltage waveform having an almost sine waveform is obtained. Then, the comparator 12 compares the output signal of each filter 10U, 10V, 10W with the virtual neutral point potential, so that a position signal of each phase having a rectangular waveform is obtained.
The control portion 7 sets a PWM duty for determining a rotation speed of the motor 3 in accordance with a control signal inputted from an external ECU (i.e., electronic control unit). Further, the control portion 7 determines a commutation timing based on the position signal detected by the position detection circuit 9, and generates a driving signal. Then, the control portion 7 outputs the driving signal to the gate driver 8.
JP-A-H07-337080 discloses a technique for starting energization at an appropriate timing when an activation instruction of a motor is given. This technique is used for a fan motor of an air-conditioner, and a rotational position of a fan in a compressor unit as an external unit is detected when the fan is rotated by catching wind.
When the motor 3 is controlled by the PWM control method without a sensor, it is necessary to remove the switching noise in the induced voltage signal by using the low pass filter 10U, 10V, 10W. As a result, a delay is generated in a phase of the induced voltage signal passing through the low pass filter 10U, 10V, 10W. To set the phase delay to be almost 90 degrees in all frequency range of the induced voltage signal, it is preferred that a CR time constant becomes larger and a cut-off frequency becomes smaller within an allowable range of decay of the signal passing through a CR filter.
In general, a device for driving a fan motor in a vehicle is mounted in an engine compartment of the vehicle. Accordingly, temperature of operating environment is disposed in an extremely wide range. The CR time constant of the low pass filter 10U, 10V, 10W has a tolerance, and, in addition, the CR time constant is easily affected by the temperature. Thus, variation of the CR time constant becomes large, and the CR time constant of each low pass filter 10U, 10V, 10W may deviate from a predetermined value.
Regarding the fan of the radiator in the vehicle, the motor 3 may start to activate from a state where the fan is rotated by catching wind when the vehicle runs. However, if the time constant of the low pass filter 10U, 10V, 10W is deviated largely, an appropriate timing is not obtained when the energization of the device starts or when the energization stops. Therefore, loss of synchronism may occur, and/or commutation pattern may not be changed, so that excess current is supplied to the device.
Further, when the motor for the vehicle is activated, it is necessary for the motor to drive normally even if a power source voltage is deviated from a predetermined value. Similar to the above case, if the time constant of the low pass filter 10U, 10V, 10W is deviated largely, an appropriate timing is not obtained when the power source voltage is rapidly changed. Therefore, loss of synchronism may occur, and/or commutation pattern may not be changed, so that excess current is supplied to the device.
Here, FIGS. 7A to 7I shows a case where the motor 3 is energized and starts to activate from a state where the fan is rotated by catching wind when the time constant of the low pass filter 10U, 10V, 10W is not deviated. Specifically, FIGS. 7A to 7I show a voltage waveform in each portion when the motor 3 is energized. FIGS. 7A to 7C show voltage waveforms of a U phase, a V phase and a W phase in the induced voltage. FIGS. 7D to 7F show voltage waveforms of a U phase, a V phase and a W phase in a signal after passing through the filter 10U, 10V, 10W. FIGS. 7G to 7I show voltage waveforms of a U phase, a V phase and a W phase in the comparator output signal. Here, the comparator output signal corresponds to the position detection signal. VIIA represents a period in which the fan is rotated by catching wind. VIIB represents a timing at which the energization starts. VIIC represents a period in which the energization is performed. In this case, the motor 3 can start to activate without difficulty.
FIGS. 8A to 8I shows a case where the motor 3 is energized and starts to activate from a state where the fan is rotated by catching wind when the time constant of the low pass filter 10U, 10V, 10W is deviated. Specifically, FIGS. 8A to 8I show a voltage waveform in each portion when the motor 3 is energized. FIGS. 8A to 8C show voltage waveforms of a U phase, a V phase and a W phase in the induced voltage. FIGS. 8D to 8F show voltage waveforms of a U phase, a V phase and a W phase in a signal after passing through the filter 10U, 10V, 10W. FIGS. 8G to 81 show voltage waveforms of a U phase, a V phase and a W phase in the comparator output signal. VIIIA represents a period in which the fan is rotated by catching wind, VIIIB represents a timing at which the energization starts, and VIIIC represents a period in which the energization is performed. In this case, since the time constant of the filter 10U, 10V, 10W is deviated, the phase in the position detection signal outputted from the comparator 12 is deviated when the energization starts.
FIGS. 9A to 9D show actual waveforms of the U phase position detection signal and U, V and W phase voltages observed by an oscilloscope after the signal passes through the filter 10U, 10V, 10W. The observation is performed at an input terminal of the buffer amplifier 11U, 11V, 11W. FIG. 9A shows a case where the time constant of the filter 10U, 10V, 10W is not deviated when the energization starts. Specifically, FIG. 9A shows a rising waveform of each phase voltage and the U phase position detection signal. FIG. 9B shows a case where the time constant of the filter 10U, 10V, 10W is not deviated when the energization stops. Specifically, FIG. 9B shows a falling waveform of each phase voltage and the U phase position detection signal. FIG. 9C shows a case where the time constant of the filter 10U, 10V, 10W is deviated when the energization starts. Specifically, FIG. 9C shows a rising waveform of each phase voltage and the U phase position detection signal. FIG. 9D shows a case where the time constant of the filter 10U, 10V, 10W is deviated when the energization stops. Specifically, FIG. 9D shows a falling waveform of each phase voltage and the U phase position detection signal. Here, the deviation of the time constant of the filter 10U, 10V, 10W is about 30% in FIGS. 9C and 9D. IXC and IXD represent a period in which the position detection signal is not obtained since the time constant of each filter 10U, 10V, 10W is deviated.
In FIGS. 9A and 9B, the U phase position detection signal is accurately outputted. In FIGS. 9C and 9D, a rise time of each phase voltage is deviated from a rise time of a reference voltage since the virtual neutral point potential as the reference voltage of the comparator 12U, 12V, 12W is obtained from a summation of three phase induced voltages. Thus, the comparator 12U, 12V, 12W cannot compare a level between the output signal of the buffer amplifier 11U, 11V, 11W with the VNP potential. Accordingly, the U phase position detection signal is not accurately outputted.
Thus, it is required for the motor driving device with a rotor position detection circuit to provide an accurate energization timing even of a time constant of a low pass filter is deviated from a predetermined value.